Stiff cleaver finally cutting thicket (6)
I believe the answer is:
corpse
'stiff' is the definition.
('corpse' can be a synonym of 'stiff')
'cleaver finally cutting thicket' is the wordplay.
'finally' suggests the final letters.
'cutting' is an insertion indicator (some letters cut their way into a word).
'thicket' becomes 'copse' ('copse' can be a synonym of 'thicket').
The final letter of 'cleaver' is 'r'.
'r' going into 'copse' is 'CORPSE'.
(Other definitions for corpse that I've seen before include "Dead body" , "Cadaver, dead person" , "Roof beam" , "Dead bodies" .)